This invention relates to light emitting devices capable of emitting light of a plurality of colors and suitable for use as organic electro-luminescence (EL) devices, and particularly to the improvement of reflecting layers therein.
Art is known for combining reflecting layers with multi-layer dielectric films laminated in alternating layers having different refractive indexes and thereby reflecting light of specific wavelengths. In the Shingaku Giho OME 94-79 (March, 1995), pp 7-12, there is a discussion on how to emit multiple colors of light using a micro resonance structure based on such a multi-layer dielectric film. According to this literature, by adjusting the positions of the light emitting layers and the reflecting layers where reflection occurs in the micro resonance structure, it is possible to output resonant light of any wavelength contained in the light emitting layers.
In Japanese Patent Application Laid-Open No. H6-275381/1994 (gazette), for example, a light emitting device having the laminar structure diagrammed in FIG. 9 is set forth. This light emitting device comprises a transparent substrate 100, a micro resonance structure 102, a positive electrode 103, a hole transport layer 104, an organic electro-luminescence (EL) layer 105, and a negative electrode 106. Of these, the thickness of the positive electrode 103 is varied respectively to select the wavelength of the light that resonates. Aluminum or alkali metals are used as the material for the negative electrode.
In a conventional electro-luminescence device, the negative electrode is ideally designed so that it completely reflects light. In actual practice, the negative electrode has been designed at times so that it is made as thin as possible to make the relative drive resistance of the EL-layer smaller.
When the negative electrode is formed thinly, however, the reflectance thereof is not always sufficient, whereupon some of the light leaks out to the back side of the electro-luminescence device without being reflected. Light utilization has thus been rather low compared to the ideal reflecting layer where complete reflection is assumed. When a mirror formed by a micro resonance structure such as cited in Japanese Patent Application No. H6-275381/1994 is positioned on the front surface (light output side) of the EL layer and wavelength selectivity thereby raised, the amount of light returning to the light emitting layer side from this mirror is increased. In a conventional device having such a structure as this, the reflectance of the negative electrode at the back surface of the EL layer is low, wherefore the light utilization factor declines significantly, which is a problem.
If only light reflecting efficiency is to be considered, there are materials known which exhibit high reflectance. However, there are restrictions on the materials which can be used for the negative electrode in an electro-luminescence device, such as energy level, and it has not been possible to use negative electrodes of high reflectance in conventional devices.
Returning the light that leaks out with a reflecting mirror is conceivable, but no suitable reflecting mirror has been devised that is suitable for a thin-film device.
A first object of the present invention is to provide a multiple-wavelength light emitting device that can emit light in a plurality of wavelengths with higher efficiency than conventionally.
A second object of the present invention is to provide a multiple-wavelength light emitting device that has higher efficiency than conventionally for multiple wavelengths and that has a simpler structure.
A third object of the present invention is to provide an electronic apparatus that can emit light in a plurality of wavelengths with higher efficiency than conventionally.
The present invention is a multiple-wavelength light emitting device for emitting light in a plurality of different wavelengths, comprising:
(1) light emission means for emitting light containing wavelength components to be output,
(2) a semi-transparent layer that transmits at least a portion of the light, placed at the back surface of the light emission means,
(3) a reflecting layer group provided on a first surface side of the light emission means, with the semi-transparent layer intervening, wherein reflecting layers that reflect light having specific wavelengths of the light ejected to (or transmitted towards) the first surface side from the light emission means via the semi-transparent layer are laminated in order in the direction of the light axis, which is in the direction of light advance, in correspondence with wavelengths of light to be output, and
(4) a semi-reflecting layer group provided on a second surface side in opposition to the first surface of the light emission means, wherein semi-reflecting layers that reflect a portion of light having specific wavelengths of the light ejected to the second surface side from the light emission means and transmit the remainder thereof are laminated in order in the direction of the light axis, which is in the direction of light advance, in correspondence with the wavelengths of light to be output.
In two or more light emission regions wherein the output light wavelength differs, the distance between the reflecting surface for light from the light emission means in a reflecting layer in the reflecting layer group that reflects light of the wavelength output in that light emission region and the reflecting surface for light from the light emission means in a semi-reflecting reflecting layer in the semi-reflecting layer group that reflects a portion of light of the wavelength output in that light emission region is adjusted so that it becomes a resonating optical path length for light ejected from that light emission region.
As based on the configuration described in the foregoing, the light that is ejected to the second surface (back surface) from the light emission means and passes through the semi-transparent layer to leak out is reflected by the action of the reflecting layer group, again passes through the semi-transparent layer, and is ejected to the first surface (front surface) side of the light emitting device. By adjusting the distance between the semi-reflecting layer and the reflecting layer, the wavelength of the light output from that light emission region is determined. In that light emission region, other reflecting layers that are optimized for light having wavelengths other than the wavelength of the light output do no more than act equally in every light emission region as semi-transparent layers simply having a constant attenuation factor, wherefore it is possible to maintain light volume balance between light of multiple wavelengths.
The terms employed in this patent application are now defined. The term xe2x80x9clight emission meansxe2x80x9d is not limiting, but it is at least necessary that wavelength components for the light that is to be output be contained. It is desirable that the xe2x80x9creflecting layersxe2x80x9d form a flat plane, but it does not necessarily have to be a uniform plane. By xe2x80x9clight emission regionxe2x80x9d is meant a region for outputting light having some wavelength dispersion, meaning that light is output in wavelengths that differ for each light emission region. The xe2x80x9cwavelengthsxe2x80x9d include not only wavelengths in the so-called visible light region but all wavelengths of a wider range including ultraviolet and infrared radiation. xe2x80x9cReflecting layersxe2x80x9d include such structures as simple completely reflecting mirrors, half mirrors, and polarizing panels in addition to interference-causing laminar structures wherein multiple layers of film having different refractive indexes are laminated. xe2x80x9cSemi-reflecting layersxe2x80x9d include structures such as half mirrors and polarizing panels in addition to interference-causing laminar structures wherein multiple layers of film having different refractive indexes are laminated. By xe2x80x9coptical path lengthxe2x80x9d is meant a distance corresponding to the product of the refractive index and thickness of a medium.
The thickness of the semi-transparent layers described in the foregoing is adjusted so that the phase of the light that, after being reflected by the reflecting layer group, again passes through that semi-transparent layer and is ejected to (or transmitted towards) the second surface side of the light emission means coincides with the phase of the light that is directly ejected to the second surface side of the light emission means.
It is here desirable that an adjustment be made so that the following relationship is satisfied.
xcexa3(nixc2x7di)=mxc2x7xcex/2
where, in a light emission region wherein light of wavelength xcex is ejected, ni is the refractive index in each layer that exists between the point of light emission in the light emission means and the light reflecting surface of the light reflecting layer for light of wavelength xcex in the light reflecting layer group, di is the thickness thereof, and m is a natural number.
In the present invention, for example, a gap adjusting layer is provided between the semi-transparent layer and the reflecting layer group for adjusting the distance between the reflecting surface for the light from the light emission means in the light reflecting layer and the reflecting surface for the light from the light emission means in the semi-reflecting layer.
The reflecting layer group described in the foregoing is configured, for example, with multiple types of reflecting layer corresponding to the wavelengths of multiple kinds of light of different wavelength separated between the light emission regions.
Specifically, the reflecting surfaces for light from the light emission means in the reflecting layers in the reflecting layer group are located at different positions in the thickness dimension for each light emission region.
Specifically, in a light emission region where light of wavelength xcex is ejected, the distance L between a reflecting surface for light from the light emission means in the reflecting layer and the reflecting surface for light from the light emission means in the semi-reflecting layer is adjusted so that the following relationships are satisfied.
L=xcexa3di
xcexa3(nixc2x7di)=mxc2x7xcex/2
where ni is the refractive index of the i""th substance between these reflecting surfaces, di is the thickness thereof, and m is a natural number.
It is preferable that, in the reflecting layer group described above, reflecting layers reflecting light of longer wavelength are placed on the light emission means side.
When reflecting layers are configured with multi-layer dielectric films, the reflective layers making up the reflecting layer group noted above are configured so that two layers of different refractive index are alternately stacked up.
The reflecting layers are adjusted so that the following relationship is satisfied.
n1xc2x7d1≈n2xc2x7d2≈(xc2xc+m/2)xc2x7xcex
where n1 is the refractive index of one layer of the two layers having differing refractive indexes, d1 is the thickness thereof, n2 is the refractive index of the other layer, d2 is the thickness thereof, xcex is the wavelength of light reflected in the reflecting layer thereof, and m is 0 or a natural number.
The surfaces on the reflecting layer group side of the semi-transparent layers noted above are formed so that they are in the same plane in all light emission regions.
In the reflecting layer group noted above, for example, multiple types of reflecting layers corresponding to the wavelengths of light of multiple different wavelengths may be laminated uniformly without being separated between light emission regions.
In the reflecting layer group noted in the foregoing, for example, spacers are provided between the reflecting layers for adjusting the optical path length between the reflecting surface for light from the light emission means in the reflecting layer and the reflecting surface for light from the light emission means in the semi-reflecting layer.
In the reflecting layer group noted above, for example, in order to adjust the optical path length from the reflecting surface for light from the light emission means in the reflecting layers noted above to the reflecting surface for the light on the light emission means side in the semi-reflecting layers noted above, the thickness of any one layer in the laminar structure of layers having different refractive indexes configuring those reflecting layers is altered.
As one example, there are cases where a plurality of types of light emission means for emitting relatively light having a plurality of wavelengths associated with the light emission regions noted above is provided so as to correspond with those light emission regions.
As another example, there are cases where light emission means capable of emitting light having wavelength components associated with all of the light emission regions noted in the foregoing is provided in common in those light emission regions.
In one specific configuration the light emission means noted above is an organic electro-luminescence layer sandwiched between electrodes, with the electrode provided at the first surface thereof being made the semi-transparent layer noted above.
These light emission means may be provided with an electron transport layer and/or a hole transport layer.
The present invention is a multiple-wavelength light emitting device for emitting a plurality of light types having different wavelengths, comprising an organic electro-luminescence layer for emitting light containing the wavelength components to be output, and an electrode for reflecting the light ejected to (or transmitted towards) the first surface of that organic electro-luminescence layer, positioned at the first surface of the organic electro-luminescence layer. The present invention is also a multiple-wavelength light emitting device that is characterized in that the electrode is configured of one substance selected from a group made up of diamond, boron nitride, or aluminum nitride.
The present invention is an electronic apparatus comprising the multiple-wavelength light emitting device of the present invention described above.